An RFID system enables radio communications to be carried out between RFID tags each equipped with an IC chip and an RFID reader writer.
RFID tags include active-type tags each of which has a battery therein and runs from the electric power of the battery, and passive-type tags each of which receives electric power from a reader writer and runs from this electric power. While active-type tags have merits, such as a long communication range and communicative stability, as compared with passive-type tags, because they include a battery therein, active-type tags also have demerits, such as complication in their structure, enlargement of their size, and high cost. The downsizing and migration to higher performance of IC chips used for passive-type tags advance with recent improvements in semiconductor technologies, and use of passive-type tags in wide fields is expected. In the case of a passive-type tag of electromagnetic induction type which is applied to an RFID tag which operates in a frequency band, such as a long frequency band or a short frequency band, a voltage is induced in the RFID tag due to an electromagnetic induction action between a transmission antenna coil of a reader writer and an antenna coil of the RFID tag and this voltage starts the IC chip of the tag, so that communications can be carried out between them. Therefore, such an RFID tag runs only within the induction field caused by an RFID reader writer, and therefore its communication range is about tens of cm.
In the case of an RFID tag which operates in a high frequency band, such as a UHF band or a microwave band, because a radio frequency communication method is applied, and power is furnished to the IC chip of the RFID tag using an electric wave, its communication range is improved greatly and reaches about 1 to 8 m. Therefore, an RFID system having this type of RFID tags can carry out batch reading of a plurality of RFID tags, reading of a moving RFID tag, and so on, which are difficult for an RFID system having a short communication range and operating in a low frequency band or a short frequency band to carry out, and it can be therefore expected that the use of the RFID system is spread greatly among a wide of fields. Passive-type tags which operate in a high frequency band, such as a UHF band or a microwave band, are disclosed by, for example, patent reference 1 and patent reference 2.
In accordance with conventional technologies for RFID tags, there have been provided an RFID tag in which, as shown in FIG. 12 of patent reference 1 (reference numeral 66 denotes a dipole antenna and reference numeral 67 denotes an IC chip), the IC chip 67 is mounted in the dipole antenna 66, the RFID tag operating as a tag for an RFID system, and an RFID tag in which, as shown in FIG. 2 of patent reference 2 (reference numeral 13 denotes a ½-wave microstrip line resonator, reference numeral 14 denotes a dielectric substrate, and reference numeral 15 denotes a ground conductor plate), an IC chip is connected between the ½-wave microstrip line resonator 13 and the ground conductor plate 15, the RFID tag being able to be mounted or attached to a metallic object (a conductor) because the radiation characteristic of the antenna is hardly influenced even if the metallic object (the conductor) is placed on a side of the ground conductor plate 15.
In FIG. 1 of patent reference 3, an RFID tag equipped with a terminal 3 formed on a surface of a substrate 1, and an IC chip 6 placed in an IC chip placement region 9 formed in a part of the substrate 1 and connected to the terminal 3 is disclosed.
This patent reference also discloses an advantage of being able to manufacture the RFID tag with a simple structure only by machining the surface of the substrate 1 and another advantage of being able to reduce the yields and manufacturing cost of the RFID tag because it is not necessary to embed the IC chip 6 into the substrate 1 and therefore the IC chip can be mounted on the antenna's upper surface.
In addition, in FIG. 19 of patent reference 4, an RFID tag 5 equipped with a dielectric member 10, a dented portion 10b for IC chip, a film base 20, an antenna pattern 30, and an IC chip 40, in which the dented portion 10b for IC chip in which the IC chip 40 can be buried is disposed in the dielectric member 10, the IC chip 40 is buried in this dented portion 10b for IC chip, and a loop antenna constructed of the antenna pattern 30, in which the film base 20 is wound around the dielectric member 10 in such a manner that the antenna pattern 30 formed on an inner surface of the film base 20 is electrically connected to the IC chip 40, so as to suppress the reduction in the communication range also in the vicinity of an electric wave absorber is disclosed.
Furthermore, in FIG. 4 of patent reference 5, an RFID tag in which an aperture 31 via which a part of a dielectric substance 20 is exposed is formed in an antenna face 30 is disclosed.
In the disclosed RFID tag, the aperture has a pair of first slits 31a extending in parallel so as to be opposite to each other, this pair of slits 31a, and a second slit 31b making the pair of slits 31a communicate with each other, and the above-mentioned second slit 31b is placed in an intermediate part of the above-mentioned pair of first slits 31a. 
A transmission-and-reception element (an IC chip) is connected to first and second power supply points 41 and 42.    [Patent reference 1] JP,2003-249820,A (FIG. 12)    [Patent reference 2] JP,2000-332523,A (FIG. 3)    [Patent reference 3] JP,2002-197434,A (FIG. 1)    [Patent reference 4] JP,2006-53833,A (FIG. 19)    [Patent reference 5] JP,2006-237674,A (FIG. 4)
Because a conventional RFID tag (an RFID tag disclosed by patent reference 1) is constructed as mentioned above, there is a problem that in a case in which an RFID tug is attached to a conductive object (a conductor), such as a metallic object, or is mounted in the vicinity of an conductive object, the dipole antenna 1 of the tug stops operating under the influence of the conductive object, and its communication range becomes short extremely.
Although it is possible to mount an RFID tag disclosed by patent reference 2 to a metallic object (a conductor), there is a problem that the structure of the tag becomes complicated and therefore manufacturing the tag becomes difficult, and this results in increase in the manufacturing cost because the RFID tag is constructed in such a manner that an IC chip is connected between a ½-wave microstrip line resonator and a ground conductor plate and it is therefore necessary to embed the IC chip into a dielectric substrate.
In the RFID tag disclosed by patent reference 3, because the thickness of the IC chip is large as compared with the conductor thickness of the antenna pattern and that of the terminal even though the downsizing of the IC chip is made to proceed, and the IC chip is mounted on the surface of the substrate, a projection is formed on the surface of the RFID tag. Therefore, it is necessary to coat the whole mounted part or part of the IC chip to protect the IC chip, and to make the surface of the RFID tag flat, as disclosed in [0023] of patent reference 3. More specifically, there is a problem that when the antenna pattern and the IC chip are mounted in the substrate, there is a possibility that the IC chip might be damaged by a shock or the like, and it becomes difficult to print something directly on the surface (the upper surface) of the RFID tag using a label printer.
Furthermore, the problem as mentioned above arises when a film in which the antenna pattern and the IC chip is mounted is bonded to the substrate because a swelling (a projection) appears in the film due to the mounting of the IC chip.
Furthermore, a problem with the RFID tag disclosed by patent reference 4 is that although a swelling (a projection) hardly appears in the film (the film base) due to the mounting of the IC chip, in a case in which the RFID is attached to a conductive object (a conductor), such as a metallic object, or is mounted in the vicinity of an conductive object, the loop antenna stops operating under the influence of the conductive object, and the communication range becomes short extremely.
Because in the RFID tag disclosed by patent reference 5, the aperture has a pair of first slits 31a extending in parallel so as to be opposite to each other, this pair of slits 31a, and a second slit 31b making the pair of slits 31a communicate with each other, and the above-mentioned aperture 31 is constructed in such a manner that regions 36 and 37 of the antenna face 30 which are formed by the substance 20 which is exposed via the aperture 31 form a matching circuit for the transmission-and-reception element, the pair of slits 31a have a shape which is long from side to side in a lateral direction which is a direction of electric power supply, an electric field which is a cross polarization component in a longitudinal direction also occurs in the pair of slits 31a while an electric field of correct polarization in the lateral direction occurs in the second slit 31b, and therefore the gain of the correct polarization component decreases.
Furthermore, because the generated cross polarization is radiated in a direction different from the direction in which the correct polarization is expected to be radiated, there may be cases that the RFID tag communicates with a reader writer even though the tag is staying at a location where it is not desirable that the RFID tag communicates with the reader writer. It is thus difficult to mount and make use of the tag.
In addition, because in the patch antenna disclosed by patent reference 5, the slit is basically placed at a position spaced apart from the center of the antenna face 30 while the power supply points 41 and 42 are arranged in the vicinity of the center of the antenna face 30, the pattern of the correct polarization also becomes asymmetrical and this has an influence upon the symmetric property of the radiation pattern of the antenna. As can be seen from this, it is clear that in the patch antenna disclosed by patent reference 1, much of the attention is focused on the matching between the regions 36 and 37 and the transmission-and-reception element (the IC chip).
In contrast, in accordance with the invention in this application, in a case in which an RFID tag (a patch antenna) is constructed as shown in FIG. 33, because the direction of an electric field which occurs in a slot (a slit) portion matches with the direction of the electric field of the patch antenna, the cross polarization component is reduced to a substantially low value, while the pattern of the correct polarization also becomes symmetrical because the slot is basically placed in the center of the patch antenna, and therefore the symmetric property of the radiation pattern of the antenna can be enhanced. However, a problem with these RFID tags (the patch antennas) is that because the length of the slit for making the matching is determined by the use frequency and the specifications of the IC chip to be used, and the minimum size of the patch antenna is decided by the length of the slit, there is a possibility that the RFID tag cannot be mounted in a case in which the mounting place of the RFID tag is narrow.
Furthermore, in the RFID tag disclosed by patent reference 5, because the thickness of the IC chip is large as compared with the conductor thickness of the antenna pattern and that of the terminal even though the downsizing of the IC chip is made to proceed, and the IC chip is mounted on the surface of the substrate, a projection is formed on the surface of the RFID tag. Therefore, in a case in which the RFID tag is required to have the resistance to environment, it is necessary to coat the whole mounted part or part of the IC chip to protect the IC chip, and to make the surface of the RFID tag flat. More specifically, a problem is that when the antenna pattern and the IC chip are mounted in the substrate, there is a possibility that the IC chip might be damaged by a shock or the like, and it becomes difficult to print something directly on the surface (the upper surface) of the RFID tag using a label printer. In addition, when a film in which the antenna pattern and the IC chip is mounted is bonded to the substrate in order to dispose a printing surface on which a label can be printed on the surface of the RFID tag, the problem as mentioned above arises because a swelling (a projection) appears in the film due to the mounting of the IC chip.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an RFID tag having a simple structure in which an antenna pattern and an IC chip are disposed on a surface of the RFID tag, and the IC chip does not need to be buried into a dielectric substrate, the RFID tag being able to be mounted regardless of whether an object onto which the RFID tag is to be mounted is a conductive one or a non-conducting one, a method of manufacturing the RFID tag, and a method of mounting the RFID tag.
It is another object of the present invention to provide an RFID tag which can be mounted regardless of whether an object onto which the RFID tag is to be mounted is a conductive one or a non-conducting one without shortening the communication range of the tag.
It is a further object of the present invention to provide an RFID tag which prevents the possibility of breakage of an IC chip due to a shock or the like, and which makes it possible to print something thereon using a label printer.
It is a still further object of the present invention to provide an RFID tag which can improve the flexibility of the size and shape thereof and can improve the flexibility of a mounting place where the RFID tag is to be mounted.